The present invention relates generally to electroluminescent organic fibers and devices. In particular, the present invention relates to environmentally stable electroluminescent organic fibers and devices and light sources comprising such fibers or devices.
Electroluminescent (“EL”) devices, which may be classified as either organic or inorganic, are well known in graphic display and imaging art. EL devices have been produced in different shapes for many applications. Inorganic EL devices, however, typically suffer from a required high activation voltage and low brightness. On the other hand, organic EL devices (“OELDs”), which have been developed more recently, offer the benefits of lower activation voltage and higher brightness in addition to simple manufacture, and, thus, the promise of wider applications.
An OELD is typically a thin film structure formed on a substrate such as glass or transparent plastic. A light-emitting layer of a luminescent organic material and optional adjacent semiconductor layers are sandwiched between a cathode and an anode. The semiconductor layers may be either hole (positive charge)-injecting or electron (negative charge)-injecting layers. The material for the light-emitting layer may be selected from many luminescent organic materials. The light emitting layer may itself consist of multiple sublayers, each comprising a different luminescent material.
When a voltage is applied across the device, electrons are injected from the cathode into a layer of organic luminescent material. At the same time, holes (positively charges) are injected from the anode into the same layer of organic luminescent material. When the positive and negative charges meet in the layer of organic luminescent material, they recombine to form excited molecules or excitons that emit radiation (in the range from ultraviolet (“UV”) to visible wavelengths) when they decay. Thus, the OELD emits radiation by electron-hole recombination due to direct electron and hole injection into the radiation-emitting layer, rather than by excitation of activator ions by electrons, as in inorganic EL devices. The wavelength, and consequently the color, of the photons emitted by the excitons depends on the electronic properties of the organic luminescent material from which the photons are generated.
Despite the aforementioned beneficial characteristics, wider acceptance for OELDs still awaits easily implementable solutions for their susceptibility to damage resulting from exposure to the environment such that robust OELDs may be made. The organic luminescent material can be reactive to moisture and oxygen. Such a reaction may cause a reduction in the useful life of the OELD. Oxidation sensitive cathode materials such as Mg—Al, Al—Li, or Ca are especially susceptible to atmospheric oxygen, which can produce dark, non-emitting spots in oxidized regions preventing current flow therethrough.
Attempts have been made to provide sealing structures for flat OELD panel displays. For example, U.S. Pat. No. 5,757,126 discloses a method for passivating a flat OELD comprising several OELD components arranged on common plastic substrate by depositing alternating layers of a transparent polymer and a dielectric material on the substrate to improve the barrier properties thereof. The transparent polymer is chosen from the group of fluorinated polymers, parylenes, and cyclotenes. The preferred dielectric materials for this device are silicon oxides and silicon nitride. PCT application WO 00/36665 discloses an flat organic light-emitting device encapsulated by at least one barrier stack comprising at least one barrier layer and one polymer layer. The material for the barrier layer is chosen from metal oxides, metal nitrides, metal carbides, metals oxynitrides, or combinations thereof. The polymer layer is made of acrylate-containing polymers. PCT application WO 00/26973 discloses a flat panel display based on inorganic and organic EL devices wherein the display EL medium is protected from oxidation by a layer that functions as both a barrier and an electrode. The layer consists of multiple alternating sublayers of barrier materials and conductive materials. The barrier materials include organic polymers, transparent dielectric materials, transparent metal nitrides, and transparent conductive oxides.
Flat-panel geometry is useful in some applications, but is not suitable in some others, such as those calling for flexible narrow shaped lighting sources. Some cable- or fiber-shaped light-emitting devices based on inorganic electroluminescent phosphors have been proposed for these applications. However, inorganic electroluminescent devices typically require high voltage and are less energy-efficient. Attempts have been made to protect these devices by coating them with an insulating polymer layer such as silicone, as disclosed in U.S. Pat. Nos. 5,753,381 and 5,876,863. Although some polymers, including silicone, can slow a penetration of liquid water, they still have appreciable permeability for water vapor and oxygen. However, effort has not been directed to improve the resistance to environmental damage of fiber-shaped OELDs.
Therefore, there still is a continued need for flexible cable- or fiber-shaped OELDs that are less affected by the environment. In particular, it is very desirable to provide a sealing structure for inhibiting the permeation of water vapor, oxygen, and/or other reactive materials into cable- or fiber-shaped OELDs or for substantially preventing these species to reach the sensitive or reactive components of OELDs.